THUNDER VALLEY: World of Adventure Series, Book 16

Twins Jeremy and Jason Parsons are helping their grandma run the Thunder Valley Ski Lodge while their grandfather recuperates from a broken hip. When Grandma Parsons joins their grandfather at the hospital, the boys are left to take care of the lodge on their own. Strange things begin happening once Grandma leaves, though. Could it be the work of a mysterious secret society called "The Broken Tree"?